After flashing the newest firmware via dfusedemo, I had some issues with Putty not connecting. Now after some time it shows up in the device manager two times. Connecting to COM4 via Putty gives me this error:

Connection to COM5 works, though. Unfortunately the Pyboard now does not show as a USB device and I can't access the files. Any ideas?

To rule out a hardware problem boot a Live Ubuntu. It should mount the Pyboard without any drivers on the first try.
After that you know that you‘re facing one of those driver problems windows is famous for and need to install every Pyboard Driver you can get until it finally works!

I've changed the cable twice and results are the same. Nevertheless, when I start Ubuntu whith Pyboard connected, I found the following message in the Ubuntu start up process:
"usb 5-2 device descriptor read/64 , error -71"